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EWOCS-V: Is Wd1-72 a recent post-interaction WR+O binary?

C. J. K. Larkin, J. Mackey, H. Jin, A. A. C. Sander, B. Reville, K. Anastasopoulou, M. Andersen, A. Bayo, J. J. Drake, E. K. Grebel, M. G. Guarcello, T. J. Haworth, V. M. Kalari, R. R. Lefever, F. Najarro, B. W. Ritchie, E. Sabbi

TL;DR

The paper addresses the origin of Wolf-Rayet stars at Solar metallicity and whether Wd1-72’s surrounding nebulosity can arise from recent binary interaction. It combines a bespoke MESA binary-evolution track with hydrodynamic simulations (PION) of non-conservative Roche Lobe Overflow (RLOF) mass loss into the Westerlund 1 cluster wind, using a mass-loss rate of about $9\times10^{-5}\,M_{\odot}\,\mathrm{yr}^{-1}$ and an outflow speed of $20\,\mathrm{km\,s^{-1}}$. The results reproduce the observed 11 μm morphology qualitatively and suggest Wd1-72 may be roughly $10\,\text{kyr}$ post-RLOF with hydrogen depleted to $\sim1\%$, implying a second or subsequent mass-transfer episode. This work supports binary interaction as a significant channel for WR evolution and provides a potential empirical benchmark for mass-loss and mass-transfer processes in progenitors of gravitational-wave binaries.

Abstract

The evolutionary origin of Wolf-Rayet (WR) stars at Solar metallicity is unclear. Single-star evolution from massive O stars, possibly via a Luminous Blue Variable phase, is challenged by binary period distributions of different WR subtypes. Wd1-72 is a WN7b+O binary embedded in the collective wind of the Galactic young massive cluster Westerlund 1 (Wd 1). It is surrounded by highly structured nebulosity, with cometary tails pointing away from Wd 1 and quasi-spherical droplets towards it. In this letter, we demonstrate that this morphology can be qualitatively reproduced by a hydrodynamic simulation of non-conservative Roche Lobe Overflow (RLOF) mass-loss into a cluster wind. Our model is based on a detailed binary evolution track consistent with key known properties of Wd1-72. Our work suggests Wd1-72 could be only ~10 kyr post-RLOF, and the hydrogen-free nature of Wd1-72 favours this being a second or subsequent RLOF episode. Follow-up observations could make Wd1-72 a valuable benchmark for probing mass-loss and mass-transfer in forming gravitational-wave binary-progenitor systems.

EWOCS-V: Is Wd1-72 a recent post-interaction WR+O binary?

TL;DR

The paper addresses the origin of Wolf-Rayet stars at Solar metallicity and whether Wd1-72’s surrounding nebulosity can arise from recent binary interaction. It combines a bespoke MESA binary-evolution track with hydrodynamic simulations (PION) of non-conservative Roche Lobe Overflow (RLOF) mass loss into the Westerlund 1 cluster wind, using a mass-loss rate of about and an outflow speed of . The results reproduce the observed 11 μm morphology qualitatively and suggest Wd1-72 may be roughly post-RLOF with hydrogen depleted to , implying a second or subsequent mass-transfer episode. This work supports binary interaction as a significant channel for WR evolution and provides a potential empirical benchmark for mass-loss and mass-transfer processes in progenitors of gravitational-wave binaries.

Abstract

The evolutionary origin of Wolf-Rayet (WR) stars at Solar metallicity is unclear. Single-star evolution from massive O stars, possibly via a Luminous Blue Variable phase, is challenged by binary period distributions of different WR subtypes. Wd1-72 is a WN7b+O binary embedded in the collective wind of the Galactic young massive cluster Westerlund 1 (Wd 1). It is surrounded by highly structured nebulosity, with cometary tails pointing away from Wd 1 and quasi-spherical droplets towards it. In this letter, we demonstrate that this morphology can be qualitatively reproduced by a hydrodynamic simulation of non-conservative Roche Lobe Overflow (RLOF) mass-loss into a cluster wind. Our model is based on a detailed binary evolution track consistent with key known properties of Wd1-72. Our work suggests Wd1-72 could be only ~10 kyr post-RLOF, and the hydrogen-free nature of Wd1-72 favours this being a second or subsequent RLOF episode. Follow-up observations could make Wd1-72 a valuable benchmark for probing mass-loss and mass-transfer in forming gravitational-wave binary-progenitor systems.
Paper Structure (9 sections, 2 equations, 5 figures)

This paper contains 9 sections, 2 equations, 5 figures.

Table of Contents

  1. Introduction
  2. Methods
  3. MESA evolutionary track
  4. Hydrodynamic model
  5. Results
  6. Discussion & Conclusions
  7. MESA track
  8. Physical constraints on the eruption
  9. Dust emission from dense clumps

Figures (5)

  • Figure 1: Left: Evolution of $\dot{M}$, $\varv_{\infty}$, $P$, $L_{\mathrm{bol}}$ and surface H abundance in our MESA track. The green, yellow and blue panels correspond to pre-RLOF, RLOF and WR wind phases. Right: Density slices for four key stages in the evolution of our simulation, with the cluster wind moving in the $-z$ direction.
  • Figure 2: Left: JWST/MIRI F1130W observations towards Wd1-72, which is marked with an orange star. Right: Density slice from our simulation mirrored around the $z$-axis at $t = 6.795$ Myr. We highlight the regions of spheroidal droplets in red, and cometary-like droplets pointing towards Wd1-72 in pink, demonstrating the qualitative correspondence between the data and simulation.
  • Figure 3: Simulation slice of $z-$axis velocity at 6.7925 Myr.
  • Figure 4: Simulation slice of expansion velocity at 6.795 Myr.
  • Figure 5: Upper half plane: thermal dust emissivity ($\epsilon$) at at $t = 6.795$ Myr calculated with the torus radiative transfer code, calculated at $\lambda=11\,\mu$m in units of erg cm$^{-3}$ s$^{-1}$. Lower half-plane: dust temperature ($T_\mathrm{dust}$) assuming radiative equilibrium, where blue regions are dust-free and so the temperature is not defined.